Oh no no no, we can't have that (^that Pikachu gif sums up how I look perfectly). If it is a Stadium game, and they don't allow all Pokemon in, I'll bet my entire savings, all of it, that it wouldn't include any of the Pokemon I actually care about (not if picked at random/based on popularity. I got lucky with the first Pokemon Rumble on the Wii). That's why I'm typically all or nothing. Besides, who really loses in that scenario?
But if it's a fighter, then it only makes sense to limit the roster, and I'd play that regardless. Someone already mentioned Pokemon Type Wild, and that's exactly what I thought at first when I saw it. My only suggestion would be to throw in some basic stage Pokemon in the mix. Smash Bros. got Squirtle with Pokemon Trainer, and I needs to play as the cute small ones. ;_;
If it is a fighter, the roster and the mechanics are gonna be the most important thing to differenciate it from PTW. I'm already giving PTW the edge not necessarily for being a 2D hand drawn fighter (though it helps), but because it didn't take itself too seriously. Check out some of those animations! It sort of reminded me of Darkstalkers, which had great character animations and a solid system for its time (mainly Vampire Savior). I don't exactly have faith that Game Freak/whoever will be as easy going today as they were back during Pokemon Stadium 1 and 2. Some of those battle animations and gestures were very humourous and brought much needed life into the little creatures.
Part of the fun in fighting games is the fighting personalities. Being able to kick all kinds of butt is awesome, but where would Street Fighter be without someone like Dan? :O